Arc lamp cathodes



' Jap.2, 1940. PAm'soT' 2,185,679

ARC LAMP CATHODES Fi led Dec. 13, 1938 Inv'enior:

Afforn 65-9. I

Patented Jan. 2, 1940 s ARC LAMP 'cn monnsfi' Jean Parisot, Pagny surMoselle, France, assign- 01 .to Socit 'Le" Cabana-Lorraine, Genneyil'liers' (Seine), France, a French company Application December 1 3,1938, Serial No. 245,467 1 I vIn France December-22, 1931 j vThisinvention relates to cathodesior the highintensity' arc,particularly for motion-picture projection; vIt is'well-known to thoseskilled. in

the art of'zthe projection of light that the highintensity are.(so-called chiefly because of the abnormally" high operating. currentdensities used) hascharacteristics which sharply distinguish' it fromall other types of arc, and indeed. from all other types of lightsource; Until the adventof the high-intensity are there was no otherlight .source available that could even approach' the "intrinsicbrilliancy of the ordinary carbon arc.. The high-intensity arc has,however,

made "available a stable: light. source that far exceeds .the carbonarcin .brilliancy. Y

The production of the high-intensity arc is dependent upon theobservance of certain well.-v known and' well-defined conditions ofoperation and ofielectrode composition, construction and arrangement Theanode is a cored carbon hav-x'. ing a .she-llcof the purestcarbonobtainable and v relativelylarge carbon. core (usually one-halfthe diameter '.of the 'shelll'fwhich is strongly mineralised(approximately. -75% by weight"- of the. core) with rareearthfluoridesor oxides.

Such is the standard type of high-intensity arc anode commonly in use,although certain modifled compositions for the core have. been :morerecently: developed. The cathode, on the other hand, hasa small carbon.core which is unmineralised ortonly slightly mineralised, its shellconsisting of the purest carbon obtainable; When mineralised, thecathode core consists'of a' mix ture, of carbon and substances. thevapours of which are highly conductive for the arc, usually alkali.compounds. Most .frequently, silicates of potassium or sodium are chosenas the alkali compounds, or .-mixtures of these silicates with theborates or carbonates. The amount of mineralisation, calculated byweight, is a maximum of 20% of the total Weight of the, core.

Such an anode and cathode are paired, for operation Lon direct current,one customary pairing being, for instance, an anode of 8mm. diameterhavinga core 4 mm. in diameter, with a cathode of 7 mm. diameter] havinga core of diam- 'eteriin theneighbourhoodof one-quarter or onethird-oithe cathode diameter. The high-intensity arc can be produced on"alternating current by pairing two anodes as above described in-:

tended :for direct current operation, but it has been found impossibleto establish the high-imtensity arcbetween two-, of the above describedcathodes intended for direct/current operation.

Itfhas' beenfound by experience that, the paired I simple 01' ccmplexhalides.

anode and cathode give rise, vin the arc, to the formation .ofYthewrareearth carbides, and these carbides-tend-to attach themselves to thenegative point. It results from "this that the electronic emissionnecessary for starting and maintaining thearc is :no longerproduced-exclusively on the core" but takes; place both on the carbidedeposits and on the core. .SinceEthese'deposits remain liquid at theworking temperature of the are, 'the drops :f orme'd areadisplaced in anirregularmanner due to capillary action-and to new deposits ofj carbide,so that the are 'no longer retains the steadiness necessary for goodmotion picture projection. Moreover, as these deposits arenon-conductors. of electricity when once cooled, the cathode thuscontaminated doesnot permit :re ignition; after 'the arc has beenextinguished. w 1

Now, a, primaryobject of my invention isv to providean improved cathodefor the high-in tensityarc by -.means of which-theseinconveniencesareavoided.atvleast .to a much greater extent than :hasbeen the case heretofore; With this object i-n view, I have foundthatthedegree .of' mineralisation of the cathode 'coreand .the

nature of the mineralising material are important factors decisive forsuppressing the depositionof carbide'entirel-y-orat least to aninnocuous level. My experiments-haveshown that the carbide deposits onthe cathode have progressively decreased as the mineralisation-of the.core is increased, but on-theotherlhand the oxygen-cone taining alkalisalts such as those mentionedabove usually employed asumineralisi'ngmaterial become dangerous when used in toolarge a proportion, .since thesilicates and carbonates are reduced at a slight {depth in the corewhere the temperature is already suflicient to permit such reduction,and the gases which are copiously evolved as a resultof this reductionentrain fine particles of core which are inj-ur-ious to the steadinessgof thearc. I have found that this -dif-.-

ficulty-can be overcome by excluding such .salts from the em andemploying instead. alkalisa'lts whichare not liable "to reduction, forexample the Therefore, the essence of my invention, briefly stated, isto *mineralise the-core'of' the cathode with 'a controlled proportion ofa simple or comp'lexhalogenated.

alkali compound.

j Good results have V been obtained with the fluorideslchlorides,fluoborates, fluosilicates of potassium-and sodium. The corresponding.com-

pounds of the other alkali metals may likewise be used but their highprice does not recommend them except in special cases.

Since, as has already been mentioned, the carbide deposit has been foundto progressively decrease (from its greatest amount in the case of acathode with an unmineralisedcore) with increasing mineralisation, itwill be appreciated that this precludes the iixing of an absolutelycritical minimum value for the degree of mineralisation. Therefore, theapproximate lowest limit mentioned later is not to be taken asnecessarily leading to a complete elimination of the carbide deposit butrather to a limitation of the deposit to an amount which is tolerable inthat it is compatible with satisfactory operation,

Whereas with too feeble a degree of mineralisation, the formation ofthecarbide deposit-takes place to a very marked and troublesome extent,

too great a degree of mineralisation on the other hand causes troublesof another kind since the mechanical stability of the core then becomesinsufficient and there is the risk. of the sputtering of incandescentmatter,'which unfavourably afiects the steadiness of the arc.

Taking all these factors into consideration,

a suitable proportion of the mineralising ma'terial has been found to be30% ormore,and preferablyin the neighbourhood of 40%, by weight of thecore, the remainder of the core being, of.

course, carbon. Investigation has shown, however, that it is not theproportion by weight which should be more especially considered, but theatomic proportion of alkali metal to carbon. This will be appreciatedupon reflection that the varifluoride to about 2.5-7.5 atoms of carbon.In

the case of sodium fluoride the optimum proportion has been found to beabout 1 molecule of this fluoride to 5 atoms of carbon; this correspondsto a proportion by weight of about 40% sodium fluoride. Itfollows thatif an alkali metal salt with bivalent anion is used, such as potassiumfluosilicate (K'zSiFe), the correct proportion calculated according tothe preferred range above mentioned is 1 molecule of the salt to about5-15 atoms of carbomsince a molecule of the salt contains .2'ato1'ns ofalkali metal. Among all the salts which can be used some are less to berecommended than others, either because they are sensitive to moistureor because they are expensive. Nevertheless, the results are good ifother halides than those mentioned are used, for example the bromides oriodides of potassium or sodium, or the compounds of lithium, rubidium orcaesium.

As has already been explained in the foregoing, it is very difficult tofix a minimum for the proportion of the mineralising material. Therange, 1 atom of alkali metal to about 2.5-7.5

atoms of carbon, appears from experiments to be the remainder of thecore being carbon, the'deposits of carbide only occur to a slightlyobjectionable extent. With approximately 25% of A the mineralisingfluoride these deposits are very slight and innocuous' Thesedeterminations have therefore led to the conclusion that the lowestlimit is of the order of 20% by weight,

, corresponding in terms of atomic proportion, to 1 atom of alkali metalto approximately 14 atoms. of I carbon. This limit depends hardly'at allon the nature of the alkali metal compounds selected, especially since ia somewhat uncertain limit is involved,- as has already been empha'-"sized. It appears therefore that a proportion of the order of 1 atom ofalkali metal to about 15 atoms of carbon may be admitted as thepractical minimum permissible limit.

Thus, the range of proportions which I contemplate employingby myinvention is broadly 1 atom of alkali metal to about 2.5-15 atoms ofcarbon, thenarrower range of 1 atomof' alkali metal to about 2.5-7.5atoms of carbonbeing the preferred one. 1

These ranges also applyif, in placev of a single alkali metal compound,two or more of the'seare associated, whether the compounds, in questioncontain ions in common or not. In other-words, a plurality .of alkalimetal compounds of the group in question, viz., non-reducible compounds,

may be used together, but in such "a way'as to always maintain aproportion within the range, 1 atom of alkali metal to about 25-15 (andpreferably 2.5-7.5) atoms of carbon. The use of mixtures has certain.advantages since it is'thus possible to deal with anydisadvantages. of aparticular salt by diluting it with one or more others more suitablefrom the'same point of. view. For example, sodium chloride is obviouslytheieast expensive material of any in the -group, but if it is used inany appreciable quantity. it crystallizes intomore or less large,crystals which always retain small quantities of mother liquor, in-

volving the tendency to decrepitation when the carbon is heated bythearc. .Byassociating for example 1-2 molecules'of' thischloride'with'three' molecules of sodium fluoride, this disadvantage isavoided and 'at the same time the cost is kept down. I

The salts of potassium are often more advan tageous than those of sodiumbecause they. do not'tend to colour thearc yellow. However, I have foundthatthisi colourationtis .quite negligible if the sodium 'isused onlyin'the "cathode, even with the largequantities which I contemplateemploying. Nevertheless, if it be preferred the potassium salts may beused either. alone -.or,

associated with other alkali metal salts. Potassium fluoride, beingdeliquescent, is not to be recommended for usebyitself but it may beincorporated in small quantities along-with other salts such as sodiumfluoride and potassium the neighbourhood of the tip and Fig. 2'shows across-section.

The cathode comprises a shell I of commercially pure carbon and a core 2which consists of carbon and mineralising material as described in theforegoing. The core Zhas a diameter of the order of about one-quarter toone-third (say about As an,

0.2-0.35) of the diameter of the shell l. example, the preferred mode ofpreparing such a cathode, the core of which is mineralised with sodiumfluoride will nowbe described. *A tube 1 of commercially pure carbon. isfirst prepared,

'-which will constitute the'shelf'of the cathode. This tube ismanufactured in known manner by extrusion from a paste of powderedcarbon agglomerated with coal tar, the soft tube formed being baked in aneutral or reducing atmosphere in order to carbonise the tar. The tube lis made of suitable externaldi'a'rneter ('7 mm; being one size normallyused) and lengthjfor use,-

the internal diameter 'ofIthe tubes being .made equal to that of thecorewhich it is to contain, for example 1.5 to.2 mmyin the case of a 7mm, cathode.

1 pressure is-injected a thickpaste containing 40% of sodium fluorideand 60% of powdered carbon, by'weight; This paste is agglomerated bymeans of a small quantity of organic binder such as gumarabic, dextrin,various. glucidic materials or suitable mixtures of these differentsubstances. The necessary quantity of water for forminga paste ofsuitable consistency is added,

the amount of water depending especially on the particle size ofthepow'dered carbon. ,When the tube i has been filled with this paste itis heated in'an oven to a suitable temperature so as to dry and hardenthe paste and char the ori ganic binder suiliciently to avoid anyevolution of tar or odours when the carbon is in use. Thus the core 2 isformed. In a modification; the core 2 is preformed from a pasteconsisting of a mixture of powdered carhem and sodium fluoridein properproportions agglomerated with a binder .of coal tar. This paste isformed into a rodwhich is hardened by baking in a neutral or reducingatmosphere. Such rod, constituting'the core 2, is introduced into thetube constituting the shell I and ce-; j mented therein, the rod and/ortube having been- I previously prepared with abinder for this purpose.The rod is made of' such diameter as to fitclosely into thetube withmild friction. Ce-

menting together of the two parts is effected by means of an organic ormineral binder, theuse of an alkali silicate having no inconveniencesunder these conditions. The binding agent is'then dried and hardened-inan .oven. 7 Of course, when following this procedure the proportion ofthe mineralising material is calculated on the weight of carbonsubsisting after baking.

Cathodes thus prepared may be utilised as such, or. more often afterhaving been jacketted with a metal conductor,-preferably copper, whichenables them to withstandwithout excessive wear the high currentdensities usual for'thehighintensity arc; -75 amps. for a 7 mm. cathode.

' I claim: 1

, 1. A cathode for a high-intensity are, com? prising a shell composedentirely of carbon enclosing a core consisting essentially of carbon andat least one oxygen free alkali metal compound, the proportion of saidcompound to car- Into this tube and under suitable bon in said corebeing 1 atom of alkali metal to about 2.5-15 atoms of carbon. it

2. A cathode for a high-intensity arc, comprising a shell composedentirely of carbon enclosing an oxygen free core consisting essentiallyof carbon and at least one halogenated alkali metal compound, theproportion of said compound to 7 carbon in said core being 1 atom ofalkali metal to about 25-15 atoms of carbon.

3. A cathode for a high-intensityarc, com-v prising a shell composedentirely of carbon enclosing a core having a diameter of about 02-035that of the shell, said core being free from oxygen one halogenatedalkali metal compound, the proand consisting essentially of carbon andat least portion oi said compound to carbon in said core atoms ofcarbon.

being '1 atom of alkali metal to about 2.5-7.5

l. A cathode for a high-intensity arc, compris ing a shell composedentirely of carbon enclosing a carbon core containing a mixture ofsodium fluoride and sodium chloride, the total proportion of said sodiumcompounds to carbon in said core being 1 atom of sodium to about 2.5-15atoms of carbon and the molecular proportion of, the

being greater than that ofthe sodium fluoride sodium chloride.

5. A cathode for a high-intensity arc, cOmclosing a carboncorecontaining a mixture of sodium fluoride and sodium chloride, the totalpro-- portion of said sodium compounds to carbon in' said core being 1atom of sodium to about 2.5-

7.5 atoms of carbon and the molecular propor tion of the sodium fluoridebeing greater than that of the sodium chloride.

6. A cathode for a high-intensity are, comprising a shell composedentirely of carbon enclosing an oxygen free carbon core containing about20-40% by weight of sodium fluoride.

7. A cathode for a direct current, high intensity arc, comprising ashell composed entirely of carbon, enclosing a core consistingesentially of car- I bon mixed'with oxygen free, halogenated, alkalimetal compounds.

8. In a direct current highintensity are light having ,a. carbonanodeunineralized with rare earth compounds, whereby rare earth carbidesare formed in the arc, a carbon cathode having a core mineralized with:an oxygen free, alkali metal compound.

9. In a direct current high intensity are light having a carbon anodemineralized with rare earth compounds, whereby rare earth carbides are,formedin the arc, a carbon cathode having a core consisting essentiallyof carbon mixed with oxygen free, halogenated, alkali metal compounds,whereby the deposit of said carbides on said cathode 'is substantiallyprevented.

JEAN ARIso'I'.

